This invention relates to apparatus for the float zone refining of crystalline semiconductor materials, and more particularly to the float zone refining of large semiconductor crystals.
Float zone refining is used to convert polycrystalline material to a high quality monocrystalline rod and, simultaneously, to remove unwanted impurities from the material. In the float zone technique a narrow molten zone is caused to move slowly along the length of a vertically disposed rod of polycrystalline material. This molten zone is unsupported, being held in position only by electromagnetic, gravitational, and surface tension forces. As the molten zone moves, the material immediately behind the zone resolidifies as monocrystalline material. The monocrystalline growth is initially nucleated by a single crystal seed and then continues in a self-seeding manner. Also, as the molten zone moves, it sweeps impurities with it, leaving the material behind the zone in a more pure state.
The molten zone is caused to traverse the length of the polycrystalline rod by moving the rod vertically downward past a stationary heating means such as an rf induction coil surrounding the material. In addition to the translational motion, a rotational motion is also imparted to improve crystal perfection and uniformity.
Consider the situation after a portion of the polycrystalline material has been converted by the float zone process to monocrystalline material. At the bottom of the apparatus a clamp holds a pencil-thin seed crystal. This seed crystal necks out to some larger desired diameter of the converted monocrystalline material and this diameter is approximately maintained along the length of the material. At the top of the apparatus a second clamp holds the upper end of the polycrystalline source material. Separating the unsupported ends of the polycrystalline and monocrystalline material and in contact with each is a zone of molten material. The polycrystalline material can be securely held by the upper clamp, but the converted monocrystalline material is precariously balanced on the thin seed crystal. The upper end of the monocrystalline rod is in contact only with the molten zone.
This method is satisfactory for the zone refining of relatively small crystals, but is clearly unsatisfactory as crystals increase both in length and diameter. Increases in crystal size require that a massive monocryystalline rod and a large volume of molten material, in both translational and rotational motion, be balanced on a thin seed crystal. Any wobble or sideways motion can cause nonuniformities, crystal imperfections, or worse, can cause the molten material to spill if the motion becomes extensive enough to cause the restraining forces of the electromagnetic force and surface tension to be exceeded. Should the latter occur, the crystal would be ruined and the equipment could be damaged.
As the semiconductor industry matures, the demand grows for large quantities of crystals of increasing diameter. Accordingly, a need existed for an improved method and apparatus applicable to the float zone refining of crystals of both increasing length and diameter.